1. Field of the Invention
The present invention relates to a transimpedance amplifier employing an amplifier unit that amplifies and converts a relatively weak input current into an output voltage of predetermined level (i.e., an output voltage of proportional level) without being saturated with a strong input current.
The transimpedance amplifier is installed in, for example, a magneto-optic disk unit that uses a magneto-optic disk. The disk unit has a photodiode that produces a very weak current in response to reflected light from the disk, and the transimpedance amplifier converts the weak current into an output voltage of required level, which is used to read the data out of the disk.
2. Description of the Related Art
The recording density of magneto-optic disks is being increased from single density to double density and even to quadruple density. The high-density disks involve more data per revolution and produce write and read signals with narrow intervals and at high frequencies. Increasing the density of a disk is synonymous with increasing the revolution speed (i.e., rotational speed) thereof. To handle such high-density disks, the magneto-optic disk units must employ a transimpedance amplifier capable of processing signals at high speed.
When reading data out of a magneto-optic disk, or when verifying data recorded on the disk, the disk units produce a very weak current. The transimpedance amplifier must have an amplifier unit to amplify such a weak current into an output voltage of required level. The high-density disks involve high-frequency write and read signals, and the high-frequency signals usually contain large noise. Due to the large noise, it is difficult for the amplifier unit to achieve a sufficient C/N ratio on the high-density disks.
When writing data to a magneto-optic disk, or when erasing data from the disk, the disk units produce a very strong current compared with the read or verify operation. In this case, the amplifier unit in the transimpedance amplifier is not required.
The amplifier unit, however, is continuously operated, and therefore, is saturated by the strong current produced during the write or erase operation. Once the amplifier unit is saturated, it takes given time until the amplifier unit is restored to its original function. The saturation of the amplifier unit will cause no problem if the disk is a low density type which involves a slow signal processing speed and allows the amplifier unit to return to its original performance before the next read operation starts. If the disk is of high density, however, it requires a high processing speed, and therefore, the restoration period, from saturation, becomes unnegligible.
FIGS. 1A and 1B show a standard magneto-optic disk unit employing a transimpedance amplifier.
The disk unit achieves read, write, erase, and verify operations. FIG. 1A shows the write operation to write data to a magneto-optic disk 110, and FIG. 1B shows the read operation to read data out of the disk 110.
Referring to FIG. 1A, the disk 110 is made from ferromagnetic material and has a lot of bit regions. The magnetization direction 115 of each bit region determines the data (data value of "1" or "0") in the region. The ferromagnetic material loses its magnetism when the temperature thereof is increased to its Curie temperature. When the material cools down, an electromagnet 130 applies an external magnetic field to a target region of the material, to magnetize the region according to the direction of the magnetic field.
To write data into a region on the disk 110, an objective lens 120 focuses a laser beam to the region, to increase the temperature of the region, and the electromagnet 130 sets the direction of magnetization of the region as required. Through these processes, data are written to the disk 110.
To read data out of the disk 110, a magneto-optic effect, i.e., a magneto-optic Kerr effect is used as shown in FIG. 1B. The magneto-optic Kerr effect is an effect that the polarization angle of a light beam reflected from a ferromagnetic surface changes depending on the direction of magnetization of the surface.
Referring to FIG. 1B, a semiconductor laser 175 emits a weak laser beam, which passes through two lenses 170 and 140 and hits a data recording region on the disk 110. The beam reflected from the disk 110 is again reflected by a mirror 150 and a beam splitter 160, passes through a half-wave plate 180 and a polarization beam splitter 185, and hits two photodiodes 190 and 191. These photodiodes 190 and 191 convert the light into current signals which are sent to amplifier units of respective transimpedance amplifiers (not shown). These amplifier units provide a differential voltage signal (an MO signal) according to a difference between the two current signals. The MO signal corresponds to the polarization angle of the light beam, i.e., the direction of magnetization of the region in question on the disk 110, to provide data recorded in the region.
During the write or erase operation shown in FIG. 1A, the photodiodes shown in FIG. 1B receive strong reflected beams from the disk 110 and generate very large currents of, for example, 10 to 100 microamperes compared with small currents of, for example, 100 nanoamperes to several microamperes produced during the read operation shown in FIG. 1B.
When the amplifier units in the transimpedance amplifiers receive such large input currents, the amplifier units are saturated to lose their amplification functions. If the saturated state continues until a read or verify operation starts after the write operation, data will not be read correctly. Namely, if the amplifier units are saturated, it is impossible to start the read or verify operation.
Magneto-optic disk units that handle single-density magneto-optic disks involve a slow data transfer rate, and this causes no problem even if the saturated amplifier units take given time to be restored to its original functions. Even if the saturation causes a trouble in the disk units, it may be solved by adjusting the intensity of the laser beam or by adjusting the dimensions of optical parts, such as mirrors and beam splitters, in the disk units.
These measures, however, are not applicable to magneto-optic disk units that handle double-density or quadruple-density disks because they must process the signals quickly to realize high-speed write and read operations. For these disk units, a long restoration period after saturation causes a serious problem.
As explained above, a large current to an input terminal of a transimpedance amplifier saturates the amplifier unit in the transimpedance amplifier and this temporarily stops the operation of the amplifier.
Amplifier units that are deeply saturated by a large current and take a long time to be restored to its normal functions are not suitable for magneto-optic disk units that handle high-density disks.